1. Field of the Invention
The present invention generally relates to an electric power semiconductor device for controlling a large amount of current, and more particularly, relates to an electric power semiconductor device in which an electric power semiconductor element and an inner lead are connected in pair with a plurality of bonding wires.
2. Description of the Related Art
Conventionally, in an electric power semiconductor device sealed with a mold resin, a main terminal (also referred to as “lead” or “lead frame”, hereinafter) is provided for drawing a main current from a semiconductor element such as an IC chip. The main tenninal is integrally formed of an inner lead and an outer lead, and the main terminal is screwed to an external substrate or a wiring part such as a bus bar at the external lead. Thus, the main current is controlled based on a gate voltage applied to a gate terminal from an external control circuit. In this conventional construction, the bonding wires for electrically connecting the electric power semiconductor element are formed of a plural aluminum wires each having a diameter of 100 to 500 μm, and the plurality of bonding wires are arranged to be connected to the same lead to ensure a current amount.
In a general construction of a semiconductor device such as a package sealed with a mold resin, the physical properties such as a linear expansion coefficient and Young's modulus of the mold resin is different from those of the lead frame of a metal material. Therefore, a shearing force acts on an interface between the mold resin and the lead frame through thermal cycles. In a case where the semiconductor device has a large mold resin part and requires long-time reliability, when the shearing force acts on the interface between the mold resin and the main lead terminal, a separation or abrasion may occur between these members. Especially, when the separation occurs in the vicinity of the wire bonding portion, the wire bonding portion is cracked. As a result, the wire per se may be broken.
As an effective constitution to prevent the separation at the interface, in a semiconductor device disclosed in, for example, a patent document 1, in order to ensure bonding-ability of the bonding wire, a plated film is formed on a post part which serves as a fixing portion of a bonding wire, and a through hole is formed so as to penetrate the plated film at the post part of an inner lead to which the bonding wire is fixed.
In this semiconductor device, since the through hole is formed in the plated film, an area of the interface between the plated film having primarily low adhesiveness with a resin and the mold resin can be reduced, so that the separation in the interface is reduced. The separation preventing structure as disclosed in the patent document 1 is effective for the semiconductor device having a structure of LOC (Lead On Chip) and the like, in which a driving current is small and a bonding wire having a diameter of about 50 μm or less is used at the post part having a minute area.
[Patent Document 1]
Unexamined Japanese Patent Laid-open Publication No. 11-238843 (see paragraphs 0017 to 0023, FIG. 1 and FIG. 4)
However, a large amount of driving current is necessarily used in an electric power semiconductor device, and metal wires each having a diameter of 100 to 500 μm are generally used as the bonding wires, and it is necessary to fix the plural number of bonding wires to one inner lead. Therefore, when the above conventional constitution is used, there arises a problem that a capacity for current density of the lead is limited because of formation of the through hole. Thus, there has been increased a demand for further improvement regarding the configuration and the arrangement of the through hole.
Moreover, in order to allow a large amount of current to be used in the electric power semiconductor device, there is employed a method fixing a bonding wire having a large diameter for the semiconductor device by applying high supersonic energy thereto. Therefore, high rigidity and preferable restraint are required for a member which fixes the bonding wire on the inner lead. When the rigidity is low and the binding force is not sufficient in the fixing portion, the fixing portion resonates with the supersonic vibration at the time of fixing, and therefore the supersonic energy could not be efficiently applied to the fixing portion to be a problem.
Especially, referring to the inner lead, it is necessary to reduce a thickness of the lead plate because of a process for forming its configuration and in view of demand for miniaturization. Moreover, when the inner lead is fixed by applying only upward and downward force, the bonding force in the surface direction becomes insufficient, to be a problem in view of manufacture and stability thereof.